1. Field of the Invention
The present invention relates to an optical modulation apparatus, and more particularly, to an optical modulation apparatus in which an operation point can be set at a stable point in intensity-modulating a light signal in accordance with an input electric signal.
2. Description of Related Art
As an optical modulator for intensity-modulating a light signal in accordance with an input data signal are modulators using effects such as photo-electric effect, photo-electro-magnetic effect and photoacoustic effect. In each of these modulators an operation point of an optical modulator is defined by applying a predetermined bias voltage to the optical modulator. Since the optical modulation apparatus changes in modulation characteristics due to peripheral temperature and degradation as time, the operation point is controlled by changing a bias voltage applied to the optical modulator. If the operation point shifts, an optimal dynamic range cannot be only obtained but the output light from the optical modulator is also distorted. In the optical modulation apparatus, the shift of the operation point is typically detected from the output light of the optical modulator to stabilize the operation point by changing the bias voltage in a direction in which the shift is cancelled.
FIG. 1 is a diagram showing the outlined structure of an optical modulation apparatus which is conventionally used. Light 22 outputted from a laser diode 21 is inputted to an optical modulator 23. A drive amplifier 24 is supplied with a data signal 25 and a sine wave signal 26 outputted from a low frequency oscillator 106. The drive amplifier 24 amplifies the data signal 25 and outputs a modulation signal 27 in which the sine wave signal 26 is superposed on the data signal 27. The optical modulator 23 modulates the intensity of light 22 emitted from the laser diode 21 in accordance with the modulation signal 27 from the drive amplifier 24. The output light of the optical modulator 23 is supplied to an optical coupler 28. The optical coupler 28 branches the inputted light into two with a predetermined branch ratio. One light component branched by the optical coupler 28 is externally taken out as the output light of the optical modulation apparatus, whereas, the other light component is supplied an light receiving element 29. The light receiving element 29 is typically a PIN diode and outputs an electric signal in accordance with the light intensity. The electric signal is supplied from the light receiving element 29 to a band pass filter 12. The band pass filter 12 extracts a frequency component of the electric signal having the same frequency as the output signal 26 from the low frequency oscillator 15. The output of band pass filter 12 is supplied to a phase detector 14. Also, the phase detector 14 is supplied with the sine wave signal 26 outputted from the low frequency oscillator 15. The phase detector 14 compares these signals in phase and outputs a bias voltage control signal 31 in accordance with the phase difference. The bias voltage control signal 31 is supplied to a DC amplifier 32. The DC amplifier 32 changes based on the bias voltage control signal 31 a voltage value of a bias voltage signal 33 which is supplied to the optical modulator 23.
FIG. 3 is a diagram showing the modulation characteristic of the optical modulator. The abscissa represents an input voltage to the optical modulator and the ordinate represents light intensity outputted from the optical modulator when light having a predetermined intensity is inputted. The optical modulator 23 shown in FIG. 1 has the modulation characteristic in which a sine wave is repeated, as shown in the figure by a solid line 41. The modulation characteristic is shifted due to the temperate change and degradation as time, as shown in the figure by a dashed line 42. The point on which a dynamic range is widest is preferable as the operation point of the optical modulator 23. Such a point is referred to as a stable point hereinafter. The black circles 43 to 45 shown in the figure represent the stable points in the modulation characteristic shown by the solid line 41. For instance, a stable point corresponding to the stable point 43 is the black circle 46 on the modulation characteristic curve shifted as shown by the dashed line 42. When the operation point is just coincident with one of the these stable points, two signal inputted to the phase detector 14 are equal to each other in phase. That is, the sine wave signal outputted from the low frequency oscillator 15 is equal to the electric signal from the band pass filter 12 in phase. If the operation point is displaced from the stable point, the phase difference is caused in accordance with the displacement. The direction of the phase difference also changes in accordance with the direction in which the operation point is displaced from the stable point. The optical modulation apparatus detects the phase difference by the phase detector 14 and holds the operation point at the stable point by controlling the bias voltage such that there is no phase difference.
An optical modulation apparatus is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Tokukaihei 5-323245) in which light intensity is modulated in accordance with a modulation signal in which a sine wave signal having a frequency sufficiently lower than an information signal is superposed on the information signal and an operation point is stabilized based on the light intensity after the modulation. In this optical modulation apparatus, an electric signal obtained by photoelectric converting the output light from the optical modulator is sample-held at the phase in which the amplitude of the superposed sine wave signal is maximum. The held voltage value is compared with a reference voltage and the magnitude and direction of change of the bias voltage are determined in accordance with the comparing result.
In the optical modulation apparatus shown in FIG. 1, the phase detector 14 typically has an allowable input voltage range. If a voltage inputted to the phase detector 14 does not fall within the allowable input voltage range, and the extracted signal is not a correct sine wave, the phase detection cannot De correctly executed. On the other hand, the magnitude and waveform of the voltage signal inputted to the phase detector 14 vary due to various factors such as change of an output level of the laser diode 21, change of passing loss in the optical modulator 23 due to degradation as time, and change of branch ratio in the optical coupler 28 due to change of peripheral temperature. For this reason, there is a problem in that the operation point goes out of the stable point so that a distortion is caused in the waveform of output light from the optical modulator. Further, if the magnitude of the signal voltage inputted to the phase detector 14 is smaller or larger than the minimum or maximum of the allowable input voltage range, the detecting precision of the phase difference becomes wrong because the ratio of signal to noise in the input signal is degraded. For this reason, there is a problem in that the operation point goes out of the stable point so that distortion is caused in the output light of the optical modulator.
In the optical modulation apparatus disclosed in JP-A-Tokukaihei 5-323245, the voltage signal is sample-held at a predetermined timing and compared with the reference voltage to detect the shift of the operation point from the stable point. Therefore, in this apparatus, if the reference voltage varies, the shift of the operation point cannot be correctly detected. For this reason, there is a problem in that stabilization of the operation point against temperature change is difficult, compared to the case that the shift of the operation point is detected through phase comparison as in the optical modulating apparatus shown in FIG. 1.